In a new video for Rolling Stone, the 18-year-old said she was inspired to record the sound during a visit to Sydney two years ago.
The then 15-year-old pop star was in Australia for Laneway Festival and found herself waiting at a traffic light for a pedestrian crossing.
“My mum and I went for a walk in Sydney,” Eilish explained.
“(When) you press it, it’s like ‘doop doop’. That’s the sound that it makes when you have to wait.”
It’s a sound all Sydneysiders are familiar with as you wait for the green man. And then comes the fast, insistent beeping when pedestrians get the go-ahead.
Baffled by what she was hearing, Eilish recorded the sound on her phone.
Weeks later, she was looking through her voice memos and stumbled across the recording.
“We call that sound ‘grart’,” her brother-producer Finneas O’Connell said in the video, causing the siblings to laugh.
It gave Finneas an idea.
“The issue is if you play that unaltered it’s not the right speed,” Finneas said.
So he slowed it down to make the sound “wider”.
This distinctively Australian sound then formed part of the beat in the chorus of Bad Guy, which came out in March.
It’s not the only weird sound you can hear in the hit track if you listen closely — such as Eilish’s distinctive laugh.
Eilish admitted the chorus instrumental hook borrowed heavily from the music of video game Plants vs. Zombies and the theme tune to Disney kids show Wizards of Waverly Place.
“It’s not at all the same melodies but it’s literally Plants vs. Zombies … and Wizards of Waverly Place,” she said.
You can watch the whole Rolling Stone video below.